There was a similar thread on the devel-list earlier this year.

http://listserv.fnal.gov/scripts/wa.exe?A2=ind0903&L=scientific-linux-devel&T=0&P=1691

John Summerfield<[log in to unmask]> wrote:
>>>> It seems that removing this isn't so straightforward either:

The easiest way to disable yum-autoupdate completely is:

     chckconfig yum off

This avoids changing the config file.

You can also exclude packages from being auto updated using 
"/etc/yum.d/yum.cron.excludes".

Stephan Wiesand wrote:
>> Connie, please defer changes in this area to SL6 at least. ...

Connie Sieh wrote:
> I have no intention of changing this behavior.  All I was going to do is 
> see if there was a easier way to remove yumautoupdate .

In the thread mentioned above, Troy explained why yum-autoupdate is 
required by yum.  However, he did add the package "yum-cron" which also 
provides yum-autoupdate and so can be used in place.  It uses yum-shell 
to run updates as a transaction and the yum-shell script (which is 
marked as config(noreplace)) can easily be modified to customise the 
behaviour.

Kel Raywood
TRIUMF